My concurrently filed application U.S. Ser. No. 288,598 entitled Non-Contact Sensing Unit For Row Crop Harvester Guidance System discloses a guidance system for detecting the location of a harvesting machine relative to the stalks of a crop row and producing output signals for automatically steering the harvesting machine or producing steering control indications for visually indicating to an operator how he should steer the harvesting machine in order to keep or bring the harvesting machine into alignment with a row of crop stalks. The guidance system includes first and second energy beam transmitters and first and second receptors for receiving the energy beams transmitted by the transmitters. The transmitters and receptors are positioned on two adjacent crop row separators of the header which is mounted on the harvesting machine ahead of its cutting mechanism. The transmitters and receptors are mounted such that the energy beams emitted by the two transmitters intersect in a region midway between the crop row separators whereby, if the two beams are interrupted substantially simultaneously by a crop stalk, the harvesting machine is aligned with the crop row. If one beam is intercepted before the other it is an indication that the harvesting machine is not aligned with the crop row. A time comparison circuit is responsive to output signals from the beam receptors to produce steering control signals indicating how the harvesting machine must be steered in order to bring it into alignment with the row of crop stalks.
As explained in the application U.S. Ser. No. 288,598 the lower leaves of some stalk crops pass through the energy beams. Where two transmitters transmit intersecting energy beams to two receptors these leaves cause false indications of stalks, and cause the guidance system to fail to sense some stalks. To overcome this problem the related application discloses that plural transmitter/receptor pairs may be provided with the two beams of each transmitter/receptor pair intersecting in a region midway between two dividers and the points of intersection of the two transmitter/receptor pairs lying in a straight line extending vertically midway between the two dividers.
U.S. application Ser. No. 288,360 discloses an alternative method and apparatus for discriminating between leaves and crop stalks. As disclosed therein, the beams transmitted by the transmitters are infra-red energy beams of sufficient intensity, and the receptors are infra-red energy sensors have a desired degree of sensitivity, the stalks intercept the beams but the beams "burn through", that is, they are sensed by the receptors through the leaves. This permits a single transmitter/receptor pair to distinguish between leaves and crop stalks.
U.S. application Ser. No. 288,598 also discloses an electronic method and apparatus for discriminating between crop stalks and weed stalks based on the length of time the energy beams are interrupted by crop stalks as compared to the length of time the beams are interrupted by weed stalks which are usually smaller than the smallest crop stalk. As explained in the application, the electronic discriminator is in part dependent on the ground speed of the harvesting machine. The present invention provides an optical method and apparatus for discriminating between crop stalks and weed stalks, completely independent of the ground speed of the harvesting machine.